Critical role of a hydrogen bond in the interaction of phospholipase A2 with transition-state and substrate analogues.

The inhibition of phospholipase A2 by an amide substrate analogue, 1-hexadecylthio-2-hexadecanoyl-amino-1,2-dideoxy-sn-glycero-3-phos phocholine, and a phosphonate transition-state analogue, 1-hexadecylthio-1-deoxy-2-hexadecylphosphono-sn-glycero-3-ph osphocholine, is dramatically influenced by pH. However, these two inhibitors show opposite pH dependencies. The amide analogue acts more potently under basic conditions, whereas the phosphonate acts more potently under acidic conditions. In both cases, ligand binding is perturbed by protonation of an enzyme functional group with an apparent pKa of 6.1, which corresponds to that of a histidine residue. Thus, His-48, which has previously been implicated in catalysis, appears to be critically involved in the hydrogen bond interactions between the enzyme and these two inhibitors. The amide analogue binds most effectively to the enzyme when His-48 is deprotonated. Upon protonation of the histidine residue, the amide cannot form a critical hydrogen bond and loses its ability to interact effectively with the enzyme. In contrast, the phosphonate analogue binds much tighter to the protonated form of the enzyme than to the deprotonated form. The phosphonate analogue needs a bridging hydrogen between the oxygen on its phosphorus atom and the N delta 1 of His-48 to form a strong hydrogen bond. At optimal pH values for inhibitor binding, both the amide and the phosphonate analogues are potent competitive inhibitors of cobra (Naja naja naja) venom phospholipase A2. The IC50 for the amide was 4.4 x 10(-4) mol fraction and for the phosphonate was 1.6 x 10(-5) mol fraction. Under the experimental conditions used, this corresponds to a bulk concentration of 2 microM and 70 nM, respectively.